Particle therapy is conducted by irradiating target tumor cells with a particle beam. Among the radiant rays used in particle therapy, x rays are most widely used. Recently, however, demand has been rising for particle therapy in which particle rays (ion beam), typically a proton beam or a carbon ion beam capable of achieving high target dose conformity, are used.
In particle therapy, excessive irradiation or inadequate irradiation may cause adverse effects on normal tissues or may lead to recurrence of a tumor. It is therefore required to irradiate a target tumor region with an ion beam for a specified dose with maximum accuracy and conformity. In the field of particle therapy, use of a scanning irradiation method has been increasing so as to realize high dose conformity. In a scanning irradiation method, a fine ion beam is used to completely irradiate the inside of a tumor to achieve a high dose only on a tumor region. The scanning irradiation method does not basically require patient-specific devices such as a collimator for forming ion beam dose distribution into a tumor shape, so that it is possible to form dose distribution into various patterns.
In the scanning irradiation method, to irradiate an arbitrary position inside a tumor, it is necessary to control the depth to which an ion beam reaches (beam range) and the irradiation position on a plane perpendicular to the direction of beam travel (on a lateral plane). The range of an ion beam can be controlled by varying the beam energy using an accelerator or a range shifter. The irradiation position on a lateral plane can be arbitrarily controlled by bending the direction of beam travel using two sets of scanning magnets.
In the scanning irradiation method, unlike in cases where an entire tumor is irradiated with spread x-rays at a time, divided regions of a tumor are irradiated with a beam in turn. Therefore, when a beam is irradiated to a target which moves, for example, due to respiration or heart beat, the relative distance between irradiation positions changes to differ from the distance assumed at the time of planning, possibly making a planned dose distribution unavailable. In a method used to avoid the above problem, movement of an irradiation target is observed and an ion beam is irradiated only when the target is in a specific position.
In other methods also proposed, reducing the difference between a planned dose distribution and a real dose distribution is attempted by controlling the number of times of irradiation or the scanning path. In the method proposed in Japanese Patent No. 4273502, for example, a same target position is irradiated plural number of times so as to average dose errors caused by movement of the target and thereby reduce the dose distribution error relative to a planned dose distribution. Furthermore, according to non-patent literature (S Water, R Kreuger, S Zenklusen, E Hug and A J Lomax, “Tumour tracking with scanned proton beams assessing the accuracy and practicalities,” Phys. Med. Biol. 54 (2009) 6549-6563), aligning a main direction of ion beam scanning with the direction of target movement brings a real dose distribution closer to a planned dose distribution.